This invention relates to a treatment of flyash and sewage sludge, and particularly to the production of a lightweight aggregate from a mixture of flyash and sewage sludge.
Flyash is a particulate by-product produced from the burning of coal, and particularly powdered bituminous coal. Coal burning power plants typically produce very large quantities of flyash that must be disposed of in an environmentally acceptable manner. It has been known for some time that flyash can be treated to form structural products (U.S. Pat. No. 1,942,769 issued Jan. 9, 1934 to Peffer, et al.) and that usable lightweight aggregates can be formed from treated flyash (U.S. Pat. No. 2,948,848 to Dublin, Jr., et al. issued Aug. 16, 1990; U.S. Pat. No. 3,702,257 issued Nov. 7, 1972 to Koning and U.S. Pat. No. 3,765,920 issued Oct. 16, 1973 to Humphrey). In the processes for treating the flyash, a typical step involves heating a flyash mixture and the heating is often accomplished in a rotary kiln.
Sewage sludge is the by-product of the treatment of wastewater. Sludge is the settled solids accumulated and subsequently separated from the liquid stream during various phases of the wastewater treatment process. The sludge may be from primary or secondary settling, or may be waste activated sludge. The sludge may be raw sludge, digested sludge or de-watered sludge. The characteristics of the sludge will vary depending upon the stage of treatment from which is drawn and also depending upon whether it has received treatment such as by digestion. However, a common characteristic of the sludge is that it contains significant organic materials.
Sludge has been generally disposed of by incineration followed by land disposal of the inert ash or by lagooning, landfilling, spreading on land for fertilizer or soil conditioning and ocean dumping where permitted. As with flyash, sewage sludge presents a considerable problem of disposal in an economical and environmentally sound manner.
It has been proposed by Greenawalt in U.S. Pat. No. 1,895,159 issued Jan. 24, 1933 that sewage sludge can be disposed of by mixing the sludge with a water absorptive material, such as ashes. The resulting mass is then sintered on a pervious hearth to form a hard cellular cake that would be suitable for use as aggregate for concrete. Another method of disposing of sewage sludge is taught by Webster, et al. in U.S. Pat. No. 4,028,130 issued June 7, 1977 in which digested sewage sludge is mixed with lime, flyash, and either alkaline earth metal, sulfates or soil to form an air hardenable composition that can be placed in an area open to the atmosphere and which will cure over a period a time to act as a landfill or as a road base material.